Paper pulp process and apparatus



April .13, 1954 J. w. NATwlcK PAPER PULP PRocEss AND APPARATUS Filed Aug. 9, 1948 .N Xus Q NSN SN m91 Altra/ne s Patented Apr. 13, 1954 UNITED STATES Param OFFICE This invention relates to new :and :improved -methods for effectingthericontinuous digestion of fibrous materials such Vas -iWoo'd fchips and Athe `likefand apparatus therefor.

The object of this invention fis to provideza `process for the continuous digestion of :a 'iibrous material such las wood .ohips, which Will ensure thorough impregnation :of lthe t'material svvit'h` the `treating `'liquor vand substantially "completeunila'tinospheric pressures to ensure :thorough foooking ina single, rcontinuous, 4.easily controlledprocess.

Stili another object is 'to produce J high `'quality `pulp `products inl` greater yields-per poundaofuwood thanlhasfhithertozbeen accomplished.

Anotherfobject isito '.providef'a continuous aprec- `ess Ifor digesting brouslfmateralswifhch is' easily and `conmletely lcontrolled 'with a .fminimum 'of man-power `and `:which fis very leconomical Awith `respe'ct'to steam consumption.

Another objectI is to Ymaintain ffthe Vchemical 'cooking liquor in 'a relatively:highadegreeiof c'on- Tcentration so as @to considerably ishorten :the'cookingtime required. i

Still other objects r are to .feiect continuous 'removal and recovery :of1=soapvandiresinsiandwof volatiles, such as turpentine .and-.oils fandntoiconf trol obnoxious, non-condensablezgases.

One sheet of\drawings .accompanies `this ap- .plication as part thereof,l.in.which likeireference numerals indicate like parts throughout In .the drawings: p

,Figure 1 is a "diagrammatic viewoffan embodiment ofiny invention; and

Figure "2 is a "diagrammatic view showing `a modification.

Referring to Figure 110ithefdrawingsgit--will be noted that 'the 'pulp passes `A-continuously through a series of Idigesters A,LB, @,Di'and Although the lillustrated fembodiment shower-five digesters, 4it :willsloe `.understood that the number of digesters maybev-aried.

The -wood fchips ,are Ladmixed with a .fsuitable chemical `cooking eliquor `in itank The `chips should .be in i a @relatively l'fine state- .of-attrition 1to permit eiiicient `penetration. of the .1quor,..and to facilitate Lpumping into `-digester.'A. IBreferalily, the chipshave been `berized,asifor example ,by passage 'through an attrition mill. lThe uniform smallsize i the vchips ,processedin this wayne suits "in increased yields and"be`tter"qu.1ty of pulpsrproduced. .The icooking liquor `may be any :'of ithefsuitable, 'commonlyzused treating liquors :such Yas `.'suli`ite,1soda, sulfate or :lime liquors. The -admiXed-wood chips-and cooking `liquor are then ltransferredito the fr'top =of conventional :digester lA bymeansof pump lfthrough pipe 1. The pulp travels down .through fdigester A under the inlfluence-of `gravity,:and isthen pumped by pump '2 through pipe 8 :into digester iB.` The :fibrous material is subsequcntlyppassed iinsimilar maniner throughf.digestersV C, i D `iandE, which i are con- A'nected:inseriesioyrpltlesi), fland Il. APumps 3, and' similarly tierce ithe Ypulp from the `bottom `of theprecedingsdigester1toithe top of the suc- =oeeding,digester. .Thecompletely digested pulp leavesithefnaldigester 1E bytwayof pipe l2 and withithe-aidofipump Baispassed into tank i3 and .then intoffsuitableapparatus forfsubsequent con- '.ventionaltreatment, such as washing and evap- 2'0 oration.

Theelevated ltemperatures and pressures required for the digestion :process .vareprovidedini- *.tially bythe `direct Vinjection of steam under super-'atmosphericpressurefinto the inalidigester 25 .-E, byway ofinlet pipe I4 which is-conneoted to perforated .ring I5 inside ythe .digesterx In Lthis :Way :uniform distribution 4of the steam ythrough .the digester contents -is iassured. Due to this direct injectionwof steam, vthe `iinal digester is under -the highestgconditions Aof temperature `and pressure present-inthe system.

uAften-passing i through and-heating `the digester contents, .the remaining steam, lwhich `carries with .it vapor-,ized volatilematerials `such las tur pentine andioiland non-condensable gases, such as :hydrogen -suliide and .sulfur dioxide, exits throughi outlet pipe L6 and .passes .tinto separator fl'l. Condensate drainsfinto branch pipe i8 which isconnectedto condensate `header t9. `The remaining ,high temperature, -high pressure vapors and .gases `then epass through -pipe 25's `into inu direct `.heatexchanger F. Perforated lring il is positionedin the 1loottoxnpoiftion ofdigester D in thee-path of the-descending pulp. The perforations :are of afsize which permits .iniiow of the cooking liquor but ,prevents :passage theren through lof lsolid `materials. .The cooking liquor Whichstrains `into ,the ring jpasses into pipe v22 and is iforced byfpump iSv-into.indirectrheater F where `it `is 'heated tto 1a relatively :high Aterriperature .thy A.thehot vapors .and gases .from .digester E,`.and then returns .to Atligester D by .way .of pipe 2.4 whichfvconnec'ts withperforatedring 2.5 positionednlthe upper lportioniof .the -digcster vTheindirecthea't exchanger may ce of any let pipe 20 and circulates around pipes 21 and 29. Condensate leaves by Way of branch pipe 3| which drains into branch pipe I8. The uncondensed gases whichlargely comprise such noncondensable gases as hydrogen sulde, sulfur dioxide and air, leave by way of branch pipe 32 which connects with header 33.

' The heated cooking liquor` percolatesjthrough the fibrous material in digester D, Ythus maintaining the contents at an elevated temperature. The

cooking liquor vaporizes to vform steam'which, j

together with other vaporized volatile materials and non-condensable gases, maintains a positive pressure in the digester. The vapors and gases generated in this way pass out of the digester by way of separator Il into indirect heat exchanger G which heats the cooking liquor from digester C. The temperature and pressure in digester D are lower than those maintained in digester E,-duexto absorption of heat units by the pulp and by the cooking liquor in the vaporization process. The temperature and pressure are also-decreased by the condensation of steam and volatiles in lheater G.

Similarly, the contents of digesters C, B and A are heated by circulation of the cooking liquor through heaters G, I-I and I, respectively, and the yheating medium for each of these heaters is derived from the vapors and gases generated'in the subsequent vdigester in the manner aforedescribed in connection with digester D and heater F. The pressures and temperatures decrease progressively towards theforward part of the system. Preferably, the system is so regulated that thepressure in digester C is at approximately atmospheric pressure. Y

The vapors and gases from digester A pass through pipe 34 into separatoi35. Condensate from the separator passes by way of outlet pipe 36 into condensate header i8. The remaining vapors and non-condensable gases pass into an efficient condenser J of any suitable type, preferably a surface condenser equipped with a suitable ejector 31, where the water, vapor and volatiles, such as turpentine and oils, are substantially entirely condensed into liquids which proceed to condensate header I9 by way of pipe 38. Header 33 which carries the non-condensable gases from the-various heat exchangers, empties into condenser J. All of the non-condensable gases, including those from digester A,are then removed by ejector 37, and may be suitably and eliiciently disposed of, as for example, by recovery of desired components or by burning.

The removal of non-condensable gases from the heat exchangers may also be accomplished apart from the condenser. This may be desirable if the quantity of such gases is relatively high. Although my preferred embodiment shows a condenser and ejector means, it will be understood that any suitable e'vacuating means may be employed which willi accomplish my purpose.

The reduction in back pressure bycondenser J produces a vacuumv indigester A, the degree of r which may be readily controlled by regulating the conditions in the condenser and the temperature of the admixed chips and cooking liquor entering the digester. The degree of vacuum is a factor in determining the temperature of the contents in digester A, inasmuch as it affects the degree of vaporization which occurs within the digester. Digester B may be maintained in a condition of partial vacuum of lesser degree than in digester A by the action of heater I which acts as a surface condenser, inasmuch as it condenses vapors from digester B and removes noncondensable gases and entrained air. Because of the reduction in temperatures which occurs pregressively from the latter to the forward portions of the system, the temperature in digester B is lower than that in digester C where the pressure may be maintained at approximately normal pressures by proper regulation of the system. Condensation of the vapors from digester B will therefore result in subatmospheric pressures.

Flash steam which leaves the system with the lcompletely digested pulp from digester E may be Ybe recovered by returning it; to the steam generator by Way of pipe 42 controlled by valve 33.

Condensate removed via header I 9 may be suitably treated to recover turpentine, oils and the like. Y

The process as above described may be modied so as to subject the chips to different treating liquors without interrupting its continuity or disturbing the desired progression of increasing pressures and temperatures. For example, it is often desirable. to remove resins and soap forming constituents by means of suitable solvents prior to treating the chips with digesting liquors. Figure 2 shows a modification which permits initial treatment of the Wood vchips, with a solvent, and subsequent replacement of the solvent with digesting liquor. The Wood chips are admixed with a suitable solvent in tank 'I and pumpd, as aforedescribed, into the upper portion of digester A. Because of the low pressure conditions prevailing in the digester, the solvent rapidly penetrates 'the chips and dissolves such components of the wood as resins, soap forming materials and the like. The solvent and chip mixture is pumped from digester A through pipe I Il!) by means of pump II into a conventional drainer-conveyor |02. The solvent is removed through conduit ID3 and then may be recovered by re-use in the system. Similarly, the dissolved components,`

such' as the resins and soap forming materials, may be recovered and constitute valuable byproducts. The remaining brous material passes into tank H14 where it is admixed with digesting liquor which is introduced into the tank through inlet pipe H15. rIhis inlet is provided with any suitable air lock 106 to maintain the pressure in the system. `The temperature of the fresh digesting liquor is controlled to maintain the progression of temperatures and pressures in the system, as r`described above. The temperature of the liquor'should, therefore, be substantially the same as that of the `withdrawn solvent. The brous material admixed with the fresh digesting liquor is then lpumped by means of pump IUT through pipe lildntddigester B, where it is subjected to subatmospheric pressures, which ensure thorough impregnation.

vA modification, :similar to the one afcredescribed, may be employed at `any desired point in .the system to drain off one type of digesting liquor and substitute a different' type of liquor, `or to remove spent liquor and substitute fresh liquor. The desired conditions of elevated temperature and pressure may be obtained in the final digester by injecting the steam into an indirect heater through which the treating liquor from the digester is circulated. Although this involves some loss of heat units which is avoided by direct injection of the steam into the last digester, this method may be advisable where it is desired to avoid any dilution of the digester contents.

The process may be employed not only for the treatment of wood, but of `other iibrous materials, such as straw, bagasse, iiax, cotton and the like.

From the foregoing description it will be apparent that the present invention is concerned with a process for cooking fibrous material, such as wood chips, to obtain pulp, and a digesting system by which that process is carried out, wherein the chips and accompanying vcooking liquor are subjected progressively to a plurality of cooking stages ranging from subatmospheric pressures and relatively low temperatures to super-atmospheric pressures and high temperatures, the flow of the chips and cooking liquor being in one direction and the flow of the temperature and pressure controlling medium being in the opposite direction. The cooking rate may be varied by controlling the temperature `and pressure of the control medium at the point of admission, the action of the terminal condenser and ejector and the control temperature of the cooking liquor. When these factors are set, the intermediate stages of the system automatically come into balance to provide a uniform progression throughout the system. The steps of the progression necessarily will be in direct ratio to the control points at the ends of the system. The cooking rate may also be regulated by varying the rate of flow of the chips and cooking liquor through the system.

The table shows the interdependent progression of pressures within the digesters under the iniiuence of variations in the initial pressure of the steam injected into the last digester and in the degree of vacuum produced by the evacuating treating .liquor to subfatmospheric pressure results in rapid penetration of the liquor into the interior of the chips. This thorough, substantially complete, impregnation of the chips considerably shortens the necessary digestion Aperiod and ensures a uniform, high quality product since there is no overcooking With consequent weakening of the fibers at the surface of the chips, while the digesting liquor is slowly penetrating into the interior.

After leaving the evacuated portion of the system, the fibrous material and liquor continuously progress through the digesters 'under progressively increased conditions of temperature and pressure which ensure thorough, rapid and complete digestion. The rate at which the material moves through the system may be easily regulated so as to prevent either overcooking or undercooking,

, Another factor which shortens the period oi cooking by my process, is the gradual increase in concentration of the digesting liquor because of vaporiaation of fluid in succeeding digesters. The resulting reduction in fluid volume maintains the remaining active digesting chemicals in relatively high and effective concentration as the brous material progresses through the system.

The pulp product produced by my process is oi uniformly high quality. Since there is no overcooking of any of the material there is no weakening or disintegration of any of the fibers. The thorough impregnation of the material and sub sequent uniform digestion eliminates the knots and screenings which are usually present in pulp produced by the present commonly used .processes and results in greater yields per pound of wood chips than is generally obtained. The removal oi soap forming materials and volatiles such as turpentine, oils, and the like, in the course of the process, facilitates their economical recovery and results in a highly puried pulp with concomitant improved physical properties. The removal of soap forming materials is particularly advantageous since it eliminates the severe foaming which generally makes the subsequent Washing of the pulp very diilicult. The process, furthermore, permits complete control of obnoxious gases such as hydrogen sulfide which do not condense under the conditions of the process. The process, therefore, may be used without obmeans in the iirst digester. 00 jection in any vicinity.

Table Initial Steam Pressuresl Digester 501bs. lbs. 80 lbs. 801bs. l0() lbs. 100 lbs.

Prossures in Digesters 43.5`lbs.-. 30.5 lbs. 54.51bs.. 39.61bs. 15.51bs.. 10.5 lbS.... 31.51bs l5 lbs. 11.51bs 4in l5 lbs- Olb. 21bS 18.25 111.-.. 51b5 17.25 in. llln 261D llln 26111.

l The presslurcs in pounds per square inch denote the increment of positive pressure above norm The process is highly economical with respect to the utilization of steam and digesting liquor. Theheat units which are initially supplied by the direct injection of steam into the last digester, are efficiently and relatively completely utilized by the interrelated system of digesters and indirect heat exchangers. The consumption of digesting liquor is also reduced by the preliminary, thorough impregnation of the wood under vacuum and the subsequent maintenance of a relatively high concentration because of vaporization and removal of the carrying iiuid.

The process is also economical with respect to the amount of man power required. The process may be easily and completely controlled by suitable instrumentation. The apparatus itself is relatively simple to operate and maintain, since there is no need for a complicated system of valves and direct steam is injected at only one point in the system.

Although I have described my invention fully andV clearly so that it may be readily comprehended by those skilled in the art, it will be understood that changes are possible within the scope of the appended claims.

Having thus described my invention, I claim:

l. Apparatus for continuously digesting fibrous materials comprising, a series of connected digesting chambers, means for moving the fibrous material and treating liquor from each chamber into the next chamber in the series, each chamber except the last chamber being connected with means including a pump tor circulatingthe chemical treating liquor through the brous material in said chamber, exteriorly ci said chamber and back into said chamber, an indirect heater exteriorly connected with said circulating means, conduit means for conducting the vapors and gases from the next subsequent digesting chamber into said heater, said vapors and gases serving to heat said treating liquor, conduit means connected with said heater for discharging condensate formed by said vapors, independent means for heating the pulp in the last digesting chamber thereby maintaining it at an elevated temperature and at superatmospheric pressure, and evacuating means associated with the iirs't digesting chamber for reducing the pressure in said chamber.

2, Apparatus for continuously digesting brous materials comprising a series of connected digesting chambers, means for moving the fibrous material and treating liquor from each chamber into the next chamber in the series,

and gases serving to heat said treating liquor,

conduit means connected with said heater for discharging condensate formed by said vapors, independent means for heating the pulp in the last digesting chamber, thereby maintaining it at elevated temperature and super-atmospheric pressure, evacuating means connected with the rst digesting chamber for reducing the pres- 8 admixed -With said different treating liquor into the next subsequent digester in the series.

3. A process for continuously digesting wood comprising, continuously passing said wood in a i-lberized state of attrition, in admixture With chemical treating liquor through a series of connected digesters and subjecting said mixture of iiberized wood and treatingI liquor to progressively increasing temperatures and pressures which progressively increase from sub-atmospheric pressures to super-atmospheric pressures, said progressively increasing temperatures and pressures being accomplished by injecting steam under super-atmospheric pressure into the last digester, indirectly heating the treating liquor in each of the preceding digesters exteriorly of said digesters by means of the vapors and gases from the next subsequent digester, said vapors and gases being at a higher temperature than said treating liquor, and maintaining a partial va-cuum in the first digester.

Ll. A process for continuously digesting brous material comprising continuously passing f brous material in admixture with chemical treating liquor through a series of connected digesters, maintaining the last digester at a re1- atively high temperature and high pressure by means of heat from an independent source, and heating the contents of each of the preceding digesters indirectly by means of the vapors and gases from the next subsequent digester, said vapors being discharged in the form of condensate, whereby the temperature and pressure increase progressively from the first to the last digester.

5. A process for continuously digesting fibrous material comprising continuously passing fibrous material in admixture with chemical treating liquor through a series of connected digesters, maintaining the last digester at a relatively high temperature and high pressure by means of steam from an independent source, and heating the contents of each of the preceding digesters indirectly by means of the vapors and gases from the next Vsubsequent digester, said heating being accomplished by circulating the treating liquor in each of the preceding digesters exteriorly of and back into said digester and indirectly heating` the liquor While exterior of said digester by means of the vapors and gases from the next subsequent digester, said vapors being discharged in the form of condensate, whereby the temperature and pressure increase progressively from therst to the last digester.

6. A process for continuously digesting flbrous materials comprising continuously passing said fibrous material in admixture with chemical treating liquor through a series of connected digesters, maintaining the last digester under relatively high temperature and high pres-4 sure by means of heat from an independent j source, heating the contents of each of the presure in said chamber, and means for replacing the initial treating liquorr with a different treating liquor at an intermediate point in the system, said means comprising a drainage chamber having outlet means for said initial treating liquor, a conduit for discharging iibrous material in admixture with initial treating liquor from one of the digesters in said series into said drainage chamber, conveyor means for discharging brous material from said drainage chamber into a chamber having inlet means for said diilz'erent treating liquor, and pump and conduit means for discharging the brous material ceding digesters indirectly by means of the va- 7. A process for continuously digesting -V brous material comprising continuously passing fibrous material in admixture with chemical treating liquor through a lseries of vconnected,

digesters, maintaining the last digester at arelatively high temperature Aand `high f pressure by means lof steam under i pressure from 1an independent source, heating the contents of each o f the preceding digesters indirectly by means of the vapors and gases from the next subsequent digester, said heatingbeing accomplished by circulating the treatingliquor in each of the preceding digesters exterioriy of and back into said ydigester and indirectly heating the liquor while outside of said digester by means of the vapors and gases from the next subsequent digester, said vapors being discharged in the form of vcondensate, and maintaining the contents of the r:tirst digester under partial vacuum and at re1- `atively low temperature, whereby theternperature and pressure increase progressively .from

the first to the last digester.

8. A process for continuously digesting Abrousmaterials comprising continuously passing said fibrous material in admixture with chemicaltreating liquor through a series of connected dgesters, maintaining the lastdigester at re1- atively high temperature and high pressure by heating with steam under pressure from an independent source, heating the contents of each of theV preceding digesters indirectly by means of the vapors and gases from the next subsequent ,digesten said indirect heating being accomplished by circulating the treating liquor in each of said preceding digesters and the vapors and gases from the next subsequent digester through an indirect heat exchanger, each of saidpreceding digesters being connected with an individ- 'ual heat exchanger, said vaporsbeing discharged inthe form of condensate, and maintaining the contents ofthe iirstdigester under partial vacuum and at relatively low temperature, whereby the temperature and pressure increase progressively from the first to the last digester.

9. A process for continuously digesting brous materials consisting of continuously passing said fibrous material in admixture with chemical treating liquor through a series of connected digesters, maintaining the last digester at relatively high temperature and high pressure by injecting steam under pressure from an independent source, heating the contents of each of the preceding dgesters indirectly by means of the vapors and gases from the next subsequent digester, said indirect heating being accomi plished by circulating the treating liquor in each of said preceding digesters and the vapors and gases from the next subsequent digester through an indirect heat exchanger, each of said preceding digesters being connected with an individual f heat exchanger, said vapors and gases beingdischarged in the forni of condensate and uncondensed gases, and maintaining the contents of the rst digester under partial vacuum and relatively low temperature, whereby the temperature and. pressure increase progressively from the first to the last digester.

lo. The process oi claim 8 wherein the initial treating liquor is replaced with a diierent treating liquor at an intermediate point in the system while maintaining substantially the same pressure and temperature prevailing at said intermediate point in the system.

11. The process of claim 9 wherein the initial treating liquor is replaced with a different treating liquor at an intermediate point in the system, said. replacement of treating liquor being accomplished by discharging the brous material admixed with the initial treating liquor from one of the digesters in said series into a chamber,

discharging saidinitial treating iiquorgdischarg- `ingthe fibrous materialinto a second chamber, :introducing the` different treating liquor, said different treating liquor being at substantially the same temperature as the Withdrawn initial treatingliquorpand then passingthe fibrous material admixed `with the diierent treating liquor into `the subsequent digester in theseries.

k12.` A process for continuously digesting brous materials consistingof continuously passing said brous material in admixture with chemical treatingliquor-through a series of ve i connected digesters, maintaining the last digester atirelativelyfhigh temperature and high pressure :by injecting 'steam `from an independent source under a pressure f of about pounds persquare inch, heating the contents of each of 'the precedingdigesters indirectlyby means ol the vvapors and gases from the next subsequent digestensaid `indirect heating being ac- -complishedby circulating the treating liquor in reach fit-said preceding digesters and thevapors and rgases from `the next subsequent digester uum equivalent to about 11 inches of mercury,

Vwhereby the temperature and pressureincrease progressively `fromrthe first-to thelast digester. 41,3. Apparatus for continuously digesting brous `materials consistngof a series of connected digesting chambers, :pumping -means for forcingfthe materialsfrom 'eachchamberiinto the next chamber in the series, each chamber except the last chamber being. connected 'with means for circulating chemical treating liquor through the brous material in said chamber, exteriorly of said chamber and back. into said chamber, said circulating means including a pump and exterior conduit, an indirect heater exteriorly connected with said circulating means, conduit means for conducting the vapors and gases from the next subsequent digesting chamber into said heater, said vapors and gases serving to heat said treating liquor, conduit means connected with said heater for discharging condensate formed by said vapors, conduit means for discharging gases from said heaters, independent means for injecting steam under pressure into the last digesting chamber, evacuating means connected with the first digesting chamber for reducing the pressure in said chamber, and means for replacing the initial treating liquor with a different treating liquor at an intermediate point in the system, said means comprising a drainage chamber having outlet means for said initial treating liquor, a conduit for discharging brous material in admixture with initial treating liquor from one of the digesters in said series into said drainage chamber, conveyer means for discharging brous material from said drainage chamber into a chamber having inlet means for said different treating liquor, and pump and conduit means for discharging the brous material admixed with said different treating liquor into the next subsequent digester in the series.

14. Apparatus for continuously digesting 1ibrous materials comprising, a series of connected digesting chambers, means for moving the :fibrous material and chemical treating liquor from each chamber to the next chamber in w:tengan the series, each chamber except the last chamber being connected with means including a -pump for circulating the treating liquor through the brous material in said chamber, exteriorly of said chamber and back into said chamber, an indirect heater exteriorly connected with said circulating means, conduit means for conducting the vapors and gases from the next subsequent .digesting chamber into said heater, said vapors and gases serving to heat said treating liquor, conduit means connected with said heater for discharging condensate formed by said vapors, means for injecting steam under pressure into the last digesting chamber, and evacuating ber, exteriorly of said chamber and back into said chamber, said circulating means including a pump and exterior conduit, an indirect heater. exteriorly connected with said circulating means, conduit means for conducting the vapors and gases from the next subsequent digesting chamber into said heater, said vapors and gases serving to heat said treating liquor, conduit means connected with said heater for discharging condensate formed by said vapors, conduit means for discharging gases from said heater,

independent means for injecting steam under pressure into the last digesting chamber, and evacuating means associated with the rst digesting chamber for reducing the pressure in said chamber.

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3. A PROCESS FOR CONTINUOUSLY DIGESTING WOOD COMPRISING, CONTINUOUSLY PASSING SAID WOOD IN A FIBERIZED STATE OF ATTRITION, IN ADMIXTURE WITH CHEMICAL TREATING LIQUOR THROUGH A SERIES OF CONNECTED DIGESTERS AND SUBJECTING SAID MIXTURE OF FIBERIZED WOOD AND TREATING LIQUOR TO PROGRESSIVELY, INCREASING TEMPERATURES ANDPRESSURES, WHICH PROGRESSIVELY INCREASE FROM SUB-ATOMSPHERIC PRESSURES TO SUPER-ATMOSPHERIC PRESSURES, SAID PROGRESSIVELY INCREASING TEMPERATURES AND PRESSURES BEING ACCOMPLISHED BY INJECTING STEAM UNDER SUPER-ATMOSPHERIC PRESSURE INTO THE LAST DIGESTER, INDIRECTLY HEATING THE TREATING LIQUOR IN EACH OF THE PRECEDING DIGESTERS EXTERIORLY OF SAID DIGESTERS BY MEANS OF THE VAPORS AND GASES FROM THE NEXT SUBSEQUENT DIGESTER, SAID VAPORS AND GASES BEING AT A HIGHER TEMPERATURE THAN SAID TREATING LIQUOR, AND MAINTIANING A PARTIAL VACUUM IN THE FIRST DIGESTER. 